1. Field of the Invention
The invention relates to a ceramic composite, in particular for use in secondary metallurgy.
2. Description of the Background Art
There are numerous known refractory ceramic components, which are used in secondary metallurgy and serve, for example, to receive or route through metallurgical melts. These include nozzle sleeves, submerged nozzles, monoblock stoppers, distribution sleeves and the like.
A high erosion and corrosion resistance, good temperature change resistance and high service life are required of these refractory products.
Depending on the application, various ceramic materials are used. For example, submerged rods of the mentioned kind today normally consist of Al2O3 graphite materials.
In this correlation, it is known that, when casting specific sorts of steel, e.g., so-called ULC (ultra-low carbon) steels, carbon is absorbed from the refractory material by the fluid steel, and aluminum oxide is built up in the steel/refractory material contact area. This buildup, also called xe2x80x9ccloggingxe2x80x9d, can pose a serious problem, for example for low-carbonized, Al-killed steels. Adhering aluminum oxide on the somewhat cooler refractory material can clog a submerged nozzle to such an extent after a certain time as to interrupt the casting process.
These phenomena have been comprehensively described in the literature (J. Poirier in New Submerged Nozzles to Reduce Alumina Buildup in Continuous Casting, pp. 79-86).
The result is found to be that carbon-free, refractory materials can better prevent the mentioned buildup of aluminum oxide better than conventional refractory materials based on Al2O3 graphite.
U.S. Pat. No. 5,370,370 describes a refractory molded article, whose one side consists of a carbon-bound, refractory oxide material, and whose other side exhibits a carbonless, oxidized zone infiltrated with a carbonless, refractory material.
For manufacturing purposes, a carbon-containing refractory material is first molded into a submerged nozzle, for example. The outside surface is subsequently glazed. The submerged nozzle is then burned in an oxidizing atmosphere. The carbon is burned out on the unglazed side in the process, which can be limited by setting the burning duration over a specific penetration depth, e.g., 2-3 mm. This xe2x80x9coxidized zonexe2x80x9d is then infiltrated with the carbonless refractory material.
The advantage to such a submerged nozzle is that it has one layer comprised of a carbonless material in the contact area to the molten bath (inside) that largely prevents the molten bath from building up. However, the manufacture of such a molded article involves an extremely high outlay, as described.
WO 95/34395 proposes a refractory part for use in steel casting, which contains carbon throughout. To still prevent the cited buildup, it is proposed that the molded article be designed as a composite, specifically with a layer adjacent to the molten bath, which becomes gastight at temperatures exceeding 1000xc2x0 C. To this end, it is proposed that the corresponding sintering aids be added to this layer, namely in the form of fine particles ( less than 50 xcexcm).
At 4-9% w/w, the carbon content of this layer is distinctly lower than the carbon content of the adjacent layer (put at 20-30% w/w). However, this feature in itself does not reduce the danger of a buildup, which makes the other feature of sintering the corresponding layer gastight during operation highly significant. This simultaneously results in a considerable operational unreliability, since the sealed layer is only to be formed during the casting process.
DE 35 23 420 C1 describes a method, in which a slip is casted into a hollow mould formed by a basic body. When the slip is in contact with the porous sucking basic body, a layer is formed on its surface by the hollow casting method.
The refractory composite according to DE 197 27 649 A1 shows two commonly compression moulded layers.
In this respect, the object of the invention is to provide a ceramic component that is able to prevent the mentioned buildup as reliably as possible, can be manufactured in a simple procedure, and exhibits the best possible resistance to erosion, corrosion and temperature change.
It was recognized that carbon-containing and carbonless (carbon free) refractory ceramic materials can be processed into a ceramic composite that remains crack-free even during exposure to temperature loads and changes in a single manufacturing process if the C-containing first layer is compression molded, and the carbonless second layer is applied to the compression molded (pressed) layer as a monolithic layer.